Projection television systems can produce a large screen display. In this system, images on the faceplate of three primary color cathode-ray tubes are magnified on the large viewing screen. A phosphor screen in these cathode-ray tubes is excited by higher density electrons as compared to direct view type, so that the deterioration of light output from the phosphor screen is serious problem for picture quality similar to the browning of faceplate glasses. Especially green light emitting phosphors are used in the most harsh conditions of electron bombardment.Several green-emitting phosphors are particularly used for projection CRTs, for example, Zn2SiO4: M n Y3 Al5 0l2:Tb Y3(Al,Ga)5O12:Tb LaOCl:Tb LaOBr:Tb and Y2SiO5:Tb. And, it was reported that impurity ions can affect the degradation of emission in Y3Al5Ol2:Tb and Y3(Al,Ga)5Ol2:Tb.
N. Hashimoto, T. Maegawa, M. Okumura, K. Yoshizaki, K. Sato, "Degradation Origin of Green Emitting Y2SiO5: Tb Phosphors for Projection Television CRT" in Proc. IS&T 1st Color and Imaging Conf., 1993, pp 52 - 54, https://doi.org/10.2352/CIC.1993.1.1.art00012